support histograms even when we lack color coder functionality

PropertyVisualizer/PropertyVisualizer.cc

@@ -161,5 +161,5 @@ OpenMesh::Vec2f PropertyVisualizer::strToVec2f (QString str)
 
 std::unique_ptr<ACG::IColorCoder> PropertyVisualizer::buildColorCoder()
 {
-    throw std::runtime_error("Requested color coder on a Propvis that does not implement it");
+    return nullptr;
 }